Document

Root of a document AST. Carries two parallel channels: `metadata` (structured key-value data, used by frontmatter/TOML/JSON backends) and `children` (a block list, used by narrative backends such as Markdown and HTML). A single Document can be serialized by any number of format backends — each backend reads whichever channels it supports and ignores the rest. This dual-channel shape is what lets one Transformation produce both markdown-with-frontmatter and flat TOML from the same input.